Characterization of novel antibiotics of the triostin group by fast atom bombardment mass spectrometry.

The quinoxaline antibiotics are cyclic depsipeptides characterized by the presence of two heterocyclic aromatic chromophores and a cross-bridge, which is either a dithioacetal (quinomycins) or a disulphide (triostins). In the naturally-occurring members of this class the chromophores are both quinoxaline-2-carboxyl (shown as the parent acid in 1). There are natural variants in the class due to differences in the amino acid composition of the peptide. For example, triostin A contains two units of N-methylvaline whereas in triostin C these are replaced by N-;--dimethylalloisoleucine (Fig. 1). The history, biological properties and mode of action of the quinoxaline antibiotics have been thoroughly reviewed by KATAGIRI et at.') and WARING.2I Experiments involving directed biosynthesis, with Streptomyces trrostinicus, have led to the isolation of novel members of the triostin group containing various chromophores." These antibiotics have now been characterized by fast atom bombardment (FAB) mass spectrometry.--') This technique permits the determination of the molecular weights of a wide range of molecules of biological interest which are often too involatile and/or thermally labile for analysis by other mass spectrometric methods. Such information is obtained in a more facile and routine manner by FAB than by field desorption mass spectroscopy, which was used previously for the characterization of new antibiotics of the quinomycin group. 7 ~ Fig. 1. The structure of the triostin group of antibiotics.